In the manufacture of photographic emulsions, and other photographic coating compositions, a wide range of chemical agents are incorporated in the emulsion, or other coating composition, to provide particular functions and benefits. These agents, which are typically utilized in solid form, are commonly referred to as "photographic addenda", i.e., addenda useful in image-recording systems employing photographic processes. They include such materials as color couplers, sensitizing dyes, desensitizing dyes, pigments, brightening agents, developing agents, development inhibitors, antifoggants, stabilizers, hardeners, ultraviolet absorbers, photoconductors, chemical sensitizers, antistatic agents, dye-releasers, and the like.
Prior to their use in the preparation of photographic emulsions and/or other photographic coating compositions, it is frequently necessary to subject solid photographic addenda to a drying process to remove occluded organic liquid. Such organic liquid remains in and on the solid particles after such processing steps as crystallization and washing. Many common organic liquids are involved, with typical examples including acetic acid, acetone, acetonitrile, dimethylformamide, ethyl acetate, heptane, isopropyl alcohol, methanol, methylene chloride, n-propyl acetate, pyridine, toluene, triethylamine and xylenes.
Conventionally, solid photographic addenda are dried by heat or vacuum or both in equipment such as tray dryers, rotary-cone dryers or rotary processors. Each of these processes relies on evaporation of the liquid remaining in or on the solid material. Usually, heat is supplied to maintain high partial pressures of the liquid whose removal is desired, to thereby ensure adequate drying rates. Hence, effective heat transfer from the dryer to the solids is of tantamount importance for solids to be dried in this manner. Surface area to volume ratios of large-scale dryers used in production are typically much smaller than in laboratory-scale models; hence production drying times are much longer than found in the laboratory.
The morphology of the conventionally-dried product can be a very serious problem. Balling and clumping of solids in dryers is common, which is detrimental not only because this reduces drying rates, but also because the size of dried solid is often not suitable. Another operation, where the average particle size of the material is reduced so that it dissolves more rapidly in a subsequent processing step, such as preparation of a photographic dispersion, is often required.
Thermal degradation of heat-sensitive materials also occurs in conventional dryers. In this case, off-specification material is produced, and the batch must be discarded or reworked to eliminate the degradation products.
The time required for conventional drying of solid photographic addenda is often prolonged, and this adds greatly to manufacturing costs. For example, with conventional rotary-cone units, some commercially important color couplers require a drying time of as long as four weeks.
Conventional dryers typically have no built-in pollution control devices and thus require added equipment to prevent the emission of potentially hazardous solvents from the production facility.
It is toward the objective of providing a new process for the removal of occluded organic liquid from solid photographic addenda, which can be carried out easily and expeditiously, which does not impair the morphology of the product nor subject it to thermal degradation, and which avoids the emission of potentially hazardous solvents, that the present invention is directed.